Patient-controlled analgesia with and without background infusion. Analgesia assessed using the demand: delivery ratio.

Sixty adult patients following general surgical operation were treated with patient-controlled analgesia using morphine. Patients were allocated into three groups to receive: no background infusion, a 1 mg.h-1 or a 2 mg.h-1 background infusion. The other controls on the patient-controlled analgesia machine were set to allow a maximum dose of morphine of 6 mg.h-1 to each group. Analgesia was assessed after 4 and 24 h using a 100 mm horizontal visual analogue scale. The number of analgesic requests made by the patient and the number of those requests which resulted in successful deliveries was recorded. Patients who received a regimen including a background infusion had improved pain relief, particularly in the first 4 h of treatment (p < 0.05). Patients who received a background infusion of 2 mg.h-1 had an increased incidence of nausea (p < 0.05). A background infusion of 1 mg.h-1, with a 1 mg bolus dose and a 12 min lockout interval provided acceptable pain relief without excessive nausea. In all three groups the ratio of analgesic requests to successful deliveries correlated with the degree of pain reported by visual analogue score (p = 0.0001).